Apparatus and means for monitoring moments in material handling equipment

ABSTRACT

A crane with a weight-lifting boom articulates about a pivot. Moments generated about the pivot are measured by mounting a strain gauge on a stress member which transmits a force from the boom to the crane&#39;&#39;s counterweight and which is located at a fixed position relative to the pivot. An electrically connected display alarm arrangement responds to the measurements and allows monitoring of the moments.

United States Patent [191 1111 3,815,116 Fink June 4, 1974 1 APPARATUS AND MEANS FOR 3,079,080 2/1963 Mason 340/267 0 MONITORING MOMENTS [N MATERIAL 3,618,064 11/1971 Hamilton 3,638,211 1/1972 Sanchez 340/267 C [75] Inventor: William Lloyd Fink, Galveston, Tex. p i E i j h w Caldwell [73] Assignee: Tedd-Shipyards Corporation, New Ass'smm -Examme' GleP swam,

York Attorney, Agent, 0r FzrmToren, McGeady and St 221 Filed: Apr; 17, 1972 anger [21] Appl. No.: 244,721 [57] ABSTRACT A crane with a weight-lifting boom articulates about a [52] US. Cl. 340/267 C, 212/39 R, 340/285 Pi Momen generated about the pivot are mea- [51] Int. Cl. G08b 21/00, B66f 17/00 re y m unting a train gauge on a stress member [58] Field of Search 340/267 C, 285, 272; hi h r n mi a forc from the boom to the cranes 212/39 1} counterweight and which is located at a fixed position relative to the pivot. An electrically connected display 56] References Cit d alarm arrangement responds to the measurements and UNITED STATES PATENTS allows monitoring of the moments.

1,614,575 l/l927 Siebs 340/267 C 23 Claims, 8 Drawing Figures I'm/[ITS i O L; U 724M! 82/065 WV -NV F' w I 7 g mew/w l Poms: sum 74 c/acu/r 1 F02 82/065 86 MIN/FIFE M1052 I al/RC5 I i I [56% I g, cMcu/r I va' l Wit 6260M) 46! I MRMlPUF/l P 01mm J0 v MEIER p I I I 75' I HANDLING EQUIPMENT sum 1 or 3 PATENTED JUN 4 i974 FIGZ APPARATUS AND MEANS FOR MONITORING MOMENTS IN MATERIAL HANDLING EQUIPMENT BACKGROUND OF THE INVENTION This invention relates to materials handling equipment, and particularly to methods and means for monitoring the safety conditions in cranes.

The invention has more particular relevance to apparatuses and means for measuring and displaying the moments generated in a crane, and associated with the safety of the crane, while the crane is lifting a load with a boom and hook assembly.

In such cranes the boom which lifts the load is articulated vertically about a pivot. This angular change of boom position varies the lifting radius of the crane. The lifting radius may also be changed by varying the length of the boom. For these reasons the moments generated by the boom and load depend not only on the weight of the load but the variable factors which affect the lifting radius, namely the boom length and the boom angle. It is possible-that a'large moment produced by a heavy load may tip the crane. Tipping a crane can, of course, represent an extreme saftey hazard. Thus, it is important to monitor the moments generated.

In presently available or known monitoring processes and systems, separate transducers independently measure the magnitude of the load to which a crane is subjected and the magnitude of the cranes lifting radius. Electronic means calculate the product of the load and the cranes lifting radius to obtain a composite or total moment. A display device then exhibits the output of the electronic means.

The necessity of using separate transducers for measuring the load and lifting radius, and for combining the measurements, imposes undesired costs and inaccuracies on such systems.

Such processes and devices are also unable to respond readily to the moments contributed by a movable boom or complex moments resulting from lifting a load with multiple hooks. Moreover, they are unable to respond accurately to variations in the rigging of the crane.

An object of this invention is to overcome the beforementioned deficiencies.

Another object of this invention is to improve methods and means for monitoring crane moments.

Another object of the invention is to provide processes and apparatuses for directly monitoring composite or total crane moments resulting from combinations of loads and lifting radii.

Yet another object of the invention is to monitor the total torque upon a crane by measuring only a single parameter.

SUMMARY OF THE INVENTION According to a feature of the invention, the total moment generated about a pivot by the weight-lifting means of a crane is determined by measuring the stress on a member which transmits the weight-lifting force of i the crane to a counterweight on the crane and which is located at a fixed position relative to the pivot. The measured value is used as an indication of the moment to which the crane is subjected.

According to another feature of the invention, the stress is measured by a transducer which responds to the strain in the member.

According to another feature of the invention, an electrically connected display responds to the measurement and allows monitoring of the moment.

According to another feature of the invention, a single transducer is used to measure the stress which is directly proportional to the total or composite moment.

According to another feature of the invention, the

stress is measured by a strain gauge secured to a fixed structural tension member which transmits a lifting force on the crane counterweight.

According to another feature of the invention, the strain gauge is welded to an existing crane structural tension member so as not to affect the strength of the member.

By virtue of these features a single transducer is capable of responding to any combination ofv boom angle, boom weight, load, or combination of loads through a single transducer output. According to another feature of the invention, an alarm or warning circuit produces a visible or audible signal when the output of the strain gauge exceeds a predetermined threshold value. Such an alarm alerts the. crane operator of any dangerous combinations of load and boom angle regardless of the rigging or changes in the boom length.

According to another feature of the invention, the

threshold level atwhich the alarm is set to produce its signal is varied in accordance with the relationship of the superstructure of the crane relative to its base. This is particularly important when the boom is turned laterally relative to the crane s base. The latter may hold the cranemore stably in one direction than a direction transverse thereto. r

The present invention furnishes a monitor system more versatile and simple than those presently available.

The above and other features of the invention are pointed out with particularity in the claims. Other objects and advantages of the invention will become more obvious from the following detailed description when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is an elevation of a boom crane embodying features of the invention.

FIG. 2 is a detailed drawing illustrating a transducer used in the embodiment of FIG. 1.

FIG. 3 is a force diagram showing the major loads, angles, distances, and forces applied-to the crane of FIG. 1, and illustrating the manner in which a single measurement is capable of monitoring the overall moment of any-combination of boom length, weight, angle, and load or rigging.

FIG. 4 is an electrical block diagram illustrating the circuitry of the embodiment of the invention in FIG. 1.

FIG. 5 is a pictorial representation of the system components utilized in the embodiment of FIG. 1.

FIG. 6 is a block diagram of a portion of the diagram in FIG. 4 illustrating a modification of the circuit in FIG. 4 and representing another embodiment of the invention; and

FIGS. 7 and 8 are schematic representations of the crane with a quadrant sensor embodying features of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1, a crane 10 supports a load 12 from a boom l4. A boom angle line 16 extending from the boom 14 through a pulley l8 and operated from a control 20 over a pulley 22, articulates the boom 14 about a pivot 24. A boom pin forms the pivot 24. A compression leg 26 of the cranes mast supports the pulley 22. A line 28 passing over the pulleys 30, .32, 34, and 36 from the control 20 adjusts the height of the load 12 relative to the end of the boom 14.

Balancing the downward forces of the boom and other weights on the crane 10 is a counterweight38. A tension leg 40 which forms a mast with the compression leg 26 transmits the downward force of the counterweight 38 to the boom 14 mainly through the line 16.

A platform 42 rotatably supports the entire cranes structure over a tractor base 44. Thus, the mast formed by the legs 26 and 40 may be rotated together with the boom 14 relative to the tractor base 44.

A strain-gauge transducer, whose structure is shown in FIG. 2, is welded to the tension leg 40 in the location illustrated-in FIG. 1. The transducer 46 is a heavy duty device capable of being easily mounted by welding onto a structural member without disturbing the strength of the member. As shown in FIG. 2, an elongated plate 48 (about l inches X 2 inches) of the.

transducer 46 is'mounted longitudinally onthe leg 40 by two transverse welds 49 and 50. The plate 48 is subjected to strains corresponding to those in leg 40.

A sensor Tl (about 1 inch X V2 inch), whose resistance varies with the strain to which it is subjected, is secured along the plate 48 so that it is strained by the strain in the plate 48 and leg 40. An envelope 52, screwed to the plate 48, includes a base 54 with a window 56 that exposes the sensor Tll to the interior of the envelope. The envelope 52 holds a temperaturecompensating sensor T2,identical to the sensor T1 but not stressed by the plate 48. TI and T2 are used in a bridge configuration along with R! and Rz; however RI- and R2 are located within the electronics package. A signal cable 58 emerges from the envelope 52. The cable furnishes energizing current to the measuring circuit and carries the measured output signal.

As shown in FIG. I, the cable 58 terminates at an 14 relative to the tractor base 44. At the same time the.

operator moves the load '12 radially and upwardly by controlling the line 16 and drawing on the line 28. The transducer 46 measures the strain, and hence the stress, imposed on the tension member 40 by the clockwise rotational moments in the vertical plane, which the weights of the load 12 and the boom l4 transmit through the boom and the line 16 and through the tension leg 40 to the counterweight 38. The transducer 46 does not affect the strength of the crane structure but where W is the weight of the load, L is the length of the boom, B is the weight of the boom, D is the distance from the pivot 24 of the center of gravity of the boom, and 6 the angle of the boom above the horizontal.

The counterweight 38 offsets the clockwise moments. The forces F, and F furnish the physical support for the weight of the crane, including the counter weight as well as the load 12. The force produced by the clockwise moments is transmitted to the counterweight 38 through the boom angle line 16 and the tension leg 40 as a lifting force on the counterweight 38.

The counterclockwise moments M are represented by the tension T in the leg 40 acting over a ra- I dius arm X. ThusM TX.

The fixed geometry of the mast and the fixed distance of the tension leg 40 from the pivot 24 allow a single measurement porportional to the tension T to be directly proportional to the total clockwise moments as shown by the equation:

TX=WL cos0+BDcos 0 Thus,

.T ={WL/X cos 0 (BD/X) cos 0 The transducer 46 monitors the strain in the tension member by the relationship T =(WL/X cos 0 (BD/X) cos 0 @zIlM when 6 is the strain value, T is the tension force in the leg 40, M is the cross-sectional area of. the tension leg 40, and E is the elastic modulus of the tension leg material.

Since the values M and E are constant, they result in a strain value a that is linearly proportional to the .slqskwi s wetness as hewn y cos cos (I) The transducer 46 senses the strain value to produce a signal in the cable 58.

The details of the electronic circuit appear in the block diagram of FIG. 4. Here, within the transducer 46, the sensor Tl forms a strain gauge bridge with 'the sensor T2 and the resistors R1 and R2. A five-volt direct current power supply 72 in the electronic control 60applies a voltage across two opposing corners of the bridge through the cable 58. Only the sensor T1 is strained by the stress on the leg 40 and the plate 48. The sensor T2 does not Contact the plate and serves only to provide temperature compensation to the bridge 70. Unbalance of the bridge 70 produces a voltage at the remaining corners. This voltage is then applied through the cable 58 to an amplifier '74 of the control 60. 7

In the control 60, a to l5 volt direct current power supply 76 energizes the amplifier 74. The power supplies 72 and 76 are in turn energized by an external power supply such as a battery or a power cable. The meter 62 displays the output of the amplifier 74.

A warning circuit 78 and a recording circuit 80 also respond to the output of the amplifier. The warning circuit issues visual and audible alarms when the amplifier 74 produces a signal indicating that the strain in the leg 40 sensed by the sensor Tl unbalances the bridge be yond a'predetermined threshold value. The recording circuit 80 continuously records the unbalance within the bridge 70 as indicated by the amplifier 74. Thus, the recording circuit 80 produces a record corresponding to the variation in strain on the leg 40. Similarly, the

voltage display meter indicates avalue corresponding to the instantaneous strain in the leg 40. Since this strain is proportional to the total clockwise moments, the meter 62 and circuits 78 and 80 are responding continuously to moment measurements.

FIG. 5 illustrates pictorially the interconnections be tween the electrical components. Here, the cable 58 joins the transducer 46 to a cabinet 82 holding the power supplies 72 and 76, the amplifier 74, the meter 62, and the circuits 78 and 80. A battery 84 energizes the system in the cabinet 82 through a power cable 86.

Acoording to other embodiments of the invention, the transducer 46 may be otherwise located. According to still another embodiment of the invention, the transducer 46 constitutes a type other than an electrical strain gauge. The moment monitor generally is applicable to systems other than cranes without departing from the invention.

Whether any one moment is likely to result in tipping of the crane depends upon the nature of the base upon which the crane structure sits.

As shown in FIG. 1, thecrane superstructure rests on a tractor base, which viewed from above is approximately square. The warning circuit 78 is set to produce its alarm signal when, based upon prior calculations, the moment is sufficient to cause tipping of the crane about its tractor base. The display meter can be marked to indicate that the particular moment along its scale exceeds the threshold value. According to another embodiment of the invention, the meter scale is marked in percentages of the calculated tipping moment. According to another embodiment of the invention, the tractor base 42 is not square but is longer longitudinally than crosswise. Under these circumstances a lesser moment would be required to tip the crane if the superstructure is rotated so that the boom extends transverse'to the direction of the tractor base than when the boom extends longitudinally. ln this transverse direction a moment which would otherwise be quite-safe if applied in the longitudinal direction, can be hazardous. FIG. 6 illustrates means for automatically adjusting the display for the rotary position of the superstructure relative to the tractor base. In FIG. 6 a quadrant sensor 90 is connected to the amplifier 74 to vary the amplification of the amplifier 74. The quadrant sensor is mounted on the superstructure and determines the direction the boom is facingThis quadrant sensor can, forexample, be composed of a reed switch mounted on the platform 42 which responds to magnetic strips mounted above the tractor base 44. The quadrant sensor 90 increases the amplification of the amplifier 74 when the boom is directed transverse to the direction of the tractor base, or within a predetermined angular range of this trans verse condition, such as within 45 of the transverse condition. The quandrant sensor decreases the amplification of the amplifier 74 when the. boom is aligned with the tractor base, or within 45 of such alignment.

The transducer'46 constitutes a rugged device capable of prolonged use with. cranes. The entire system represents a simple measuring apparatus able to withstand the demanding conditions associated with cranes.

FIGS. 7 and 8 illustrate plan views of an embodiment of a quadrant sensor 90 mounted on the crane. When the superstructure, here designated 92, aligns the boom longitudinally with the base 42 as shown .in FIG. 7, a pair of reed switches 94 and 96 mounted under the superstructure lie between two magnetic strips on the base 42. When the boom 14 is turned as shown in F IG. 8, the reed switches overlie the magnetic strips and are energized by them. The energized reed switches increase the amplification of the amplifier 74 to cause a higher reading in circuits 78 and 80. This initiates an alarm for smaller moments when the boom 14 is aligned transverse to the base 42.

According to another embodiment of the invention, a manual switch located in the electronics control may be set by the crane operator for side positions, corner positions, forward positions, or side position with outriggers.

According to another embodiment of the invention, the magnetic strips 98 and 100 are mounted on the superstructure 92 and the switches 94 and 96 on the base 42.

It should be noted that the edge of the base 42 in the direction of the boom 14 is the tipping point of the crane. In FIG. 3 this point is approximately aligned with pivot 24. When this tipping point is not near the pivot 24 as in FIG. 8, the upward force on the tipping point produces an extra clockwise moment about point'24 in FIG. 3. This moment adds a term to the value T and to the numerator of the value c. This term is effectively entered into the amplifier by the quadrant sensor 90. An extra constant term is also added to the amplifier if the tipping point is always far from the point 24.

According to another embodiment of the invention, the amplification of the amplifier 74 is increased to embrace a safety factor. According to still another embodiment the safety factor is sufficiently large to eliminate the need for the quadrant sensor. The safety factor is, according to another embodiment, entered into the circuits 78 and 80.

While embodiments of the invention have been described in detail, it will be obvious to those skilled in the art that the invention may be embodied otherwise without departing from its spirit and scope.

What is claimed is:

l. A materials handling apparatus, comprising weight-lifting means forming a pivot about which weights applied to said weight-lifting means produce moments in one direction, counterweight means, stressable means connecting said counterweight means to said weight-lifting means at a fixed distance relative to the pivot for applying an opposing moment to said weight-lifting means. measuring means coupled to said stressable means form a mast having a compression member in said weight-lifting means and a tension I member in said stressable means.

4. An apparatus as in claim 1, wherein said stressable means is subject to strain and said measuring means measures the strain.

5. An apparatus as in claim 1, wherein said stressable means includes a tension member.

6. An apparatus as in claim 1, wherein said measuring means includes a transducer.

7. An apparatus as in claim 1, wherein said measuring means includes a strain gauge.

8. An apparatus as' in claim 1 wherein said measuring means includes a strain gauge forming a portion of a strain gauge bridge.

9. An apparatus as in claim 1 wherein said stressable means is subject to strain and said measuring means includes a transducer for measuring the strain.

10. An apparatus as in claim 1, wherein said stressable means is subject to strain and said measuring means includes a strain gauge for measuring the strain.

11. An apparatus as in claim 1-, wherein said stressable means includes a tension member forming a portion of a mast, said weight-lifting means including a compression member forming another part of said mast, said tension member being fixedly mounted relative to the pivot, said measuring means including a strain gauge forming a portion of a bridge and mounted on said tension member.

12. An apparatus as in claim 1, wherein said output means includes a display.

13. An apparatus as in claim 1, wherein said output means includes a visual alarm.

14. An apparatus as in claim 1, wherein said output means includes an audible alarm.

15. An apparatus as in claim 1, wherein said stressable means is subject to strain, and wherein said measuring means includes a strain gauge for measuring the strain in said stressable means, said output means including display means responsive to said strain gauge for displaying the strain in said stressable means.

16. An apparatus as in claim 1, wherein said weightlifting means includes' a boom, said weight-lifting means and said stressable means forming a mast, said mast having a tension member in said stressable means and a compression member in said weight-lifting means, said tension member being fixed relative to said pivot, said measuring means including a strain gauge, said output means including electrical display means coupled to said strain gauge.

17. An apparatus as in claim 1, wherein said stressable means is subject to strain, and wherein said measuring means includes a strain gauge for measuring the strain in said stressable means, said output means including visual alarm means responsive to the strain reaching a predetermined value.

18. An apparatus as in claim 1, wherein said weightlifting means includes a boom, said weight lifting means forming a portion of a mast, said mast having a tension member in said stressable means and a compression member in said weight-lifting means, said tension memberbeing fixed relative to said pivot, said measuring means including a strain gauge, said output means including visual alarm means electrically, responsive to said strain gauge for producing a visual alarm when the strain exceeds a predetermined value.

19. An apparatus as in claim 1, wherein said stressable 'means is subject to strain, and wherein said measuring means includes a strain gauge for measuring the strain in said stressable means, said output means in cluding audible alarm means responsive to the strain reaching a predetermined value.

20. An apparatus as in claim 1, wherein said weightlifting means includes a boom, and said stressable means and said weight-lifting means forming a mast, said mast having a tension member in said stressable means and a compression member in said weight-lifting means, said tension member being fixed relative to said pivot, said measuring means including a strain gauge, said output means including audible alarm means electrically responsive to said strain gauge for producing an audible alarm when the strain exceeds a predetermined value.

21. An apparatus as in claim 1, further comprising a base, said counterweight means and said stressable means and said weight-lifting means forming a superstructure rotatably mounted on said base, rotationally sensitive means coupled to said superstructure for responding to the rotational position of said superstructure relative to said base and coupled to said output means for varying the output of said output means as a function of the rotational position of the superstructure.

22. An apparatus as in claim 21 wherein said counter-weight means and said stressable means and said weight-lifting means forms a superstructure rotatably mounted on said base; rotationally sensitive means coupled to said superstructure and relative to said base and coupled to said output means for varying the output of said output means .as a function of the rotational position of thesuperstructure relative to said base.

' 23. An apparatus as in claim 21 wherein said counter-weight means and said stressable means and said weight-lifting means form a superstructure rotatably mounted on said base, rotationally sensitive means coupled to said superstructure for responding to the rotational position of said superstructure relative to said base and coupled to said output means for varying the predetermined value.

UNIT D STATES PATENT OFFICE (IER'IIFHINIE ()i (KER R E( I'II(')N Pateht No'. l r l 'Dnced June 19-74 Inventor(s) illiam Lloyd Fink It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I n the heading of the patent:

The name of the assignee should read:

:-Todd-Shipyards Corporation-fl Signed and sealed this 1st day of October 1974.

"(Sign fittest:

MCCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM po'mso no'sg) v USCOMM-DC scan-ps9 I 05. GOVERNMENT PRINTING OFFICE: 19! 0-366-334. 

1. A materials handling apparatus, comprising weight-lifting means forming a pivot about which weights applied to said weightlifting means produce moments in one direction, counterweight means, stressable means connecting said counterWeight means to said weight-lifting means at a fixed distance relative to the pivot for applying an opposing moment to said weight-lifting means, measuring means coupled to said stressable means for measuring the stress in said stressable means, and output means coupled to said measuring means for producing output signals corresponding to the stress measured.
 2. An apparatus as in claim 1, wherein said weight-lifting means includes a boom articulatable about the pivot.
 3. An apparatus as in claim 1, wherein said weight-lifting means includes a boom articulatable about the pivot, and wherein said weight-lifting means and said stressable means form a mast having a compression member in said weight-lifting means and a tension member in said stressable means.
 4. An apparatus as in claim 1, wherein said stressable means is subject to strain and said measuring means measures the strain.
 5. An apparatus as in claim 1, wherein said stressable means includes a tension member.
 6. An apparatus as in claim 1, wherein said measuring means includes a transducer.
 7. An apparatus as in claim 1, wherein said measuring means includes a strain gauge.
 8. An apparatus as in claim 1 wherein said measuring means includes a strain gauge forming a portion of a strain gauge bridge.
 9. An apparatus as in claim 1 wherein said stressable means is subject to strain and said measuring means includes a transducer for measuring the strain.
 10. An apparatus as in claim 1, wherein said stressable means is subject to strain and said measuring means includes a strain gauge for measuring the strain.
 11. An apparatus as in claim 1, wherein said stressable means includes a tension member forming a portion of a mast, said weight-lifting means including a compression member forming another part of said mast, said tension member being fixedly mounted relative to the pivot, said measuring means including a strain gauge forming a portion of a bridge and mounted on said tension member.
 12. An apparatus as in claim 1, wherein said output means includes a display.
 13. An apparatus as in claim 1, wherein said output means includes a visual alarm.
 14. An apparatus as in claim 1, wherein said output means includes an audible alarm.
 15. An apparatus as in claim 1, wherein said stressable means is subject to strain, and wherein said measuring means includes a strain gauge for measuring the strain in said stressable means, said output means including display means responsive to said strain gauge for displaying the strain in said stressable means.
 16. An apparatus as in claim 1, wherein said weight-lifting means includes a boom, said weight-lifting means and said stressable means forming a mast, said mast having a tension member in said stressable means and a compression member in said weight-lifting means, said tension member being fixed relative to said pivot, said measuring means including a strain gauge, said output means including electrical display means coupled to said strain gauge.
 17. An apparatus as in claim 1, wherein said stressable means is subject to strain, and wherein said measuring means includes a strain gauge for measuring the strain in said stressable means, said output means including visual alarm means responsive to the strain reaching a predetermined value.
 18. An apparatus as in claim 1, wherein said weight-lifting means includes a boom, said weight lifting means forming a portion of a mast, said mast having a tension member in said stressable means and a compression member in said weight-lifting means, said tension member being fixed relative to said pivot, said measuring means including a strain gauge, said output means including visual alarm means electrically responsive to said strain gauge for producing a visual alarm when the strain exceeds a predetermined value.
 19. An apparatus as in claim 1, wherein said stressable means is subject to strain, and wherein said measuring means includes a strain gauge for measuring the strain in said stressable mEans, said output means including audible alarm means responsive to the strain reaching a predetermined value.
 20. An apparatus as in claim 1, wherein said weight-lifting means includes a boom, and said stressable means and said weight-lifting means forming a mast, said mast having a tension member in said stressable means and a compression member in said weight-lifting means, said tension member being fixed relative to said pivot, said measuring means including a strain gauge, said output means including audible alarm means electrically responsive to said strain gauge for producing an audible alarm when the strain exceeds a predetermined value.
 21. An apparatus as in claim 1, further comprising a base, said counterweight means and said stressable means and said weight-lifting means forming a superstructure rotatably mounted on said base, rotationally sensitive means coupled to said superstructure for responding to the rotational position of said superstructure relative to said base and coupled to said output means for varying the output of said output means as a function of the rotational position of the superstructure.
 22. An apparatus as in claim 21 wherein said counter-weight means and said stressable means and said weight-lifting means forms a superstructure rotatably mounted on said base; rotationally sensitive means coupled to said superstructure and relative to said base and coupled to said output means for varying the output of said output means as a function of the rotational position of the superstructure relative to said base.
 23. An apparatus as in claim 21 wherein said counter-weight means and said stressable means and said weight-lifting means form a superstructure rotatably mounted on said base, rotationally sensitive means coupled to said superstructure for responding to the rotational position of said superstructure relative to said base and coupled to said output means for varying the predetermined value. 